1. Field of the Invention
This invention relates to medical devices and techniques for low-power Rf (radiofrequency) delivery to develop ionic perturbations in biologic tissue for creating localized thermal effects, such Rf energy being focused in part by simultaneous photonic tissue-stimulation, and more particularly to such a Rf/photonic system for delivering low-power focused thermal effects on subsurface tissue (e.g., on collagenous tissue in a herniated disc to shrink collagen in the annulus fibrosus to reduce the herniation).
2. Description of Related Art
The incidence of disc-related disease is about 1.7 percent of the general population, affecting an estimated 85 million people worldwide. Up to 80% of those of suffering can be treated with conservative measures such as anti-inflammatory drugs, muscle relaxing drugs, acupuncture or bed rest. The remaining 20% of the patient population may be candidates for a more aggressive therapies, which include surgeries. The primary surgical approach for a prolapsed or herniated disc has been to remove the offending disc (diskectomy). In this procedure, most or all of the prolapsed disc is resected, at times with a portion of the overlying bone. Such open surgeries have a success rate of 50%-60% or more in reducing lumbar pain. Long-term outcomes, however, are called into question because there are often degenerative changes in both the disc and surrounding bones following such a disk surgery.
The structural elements of the spinal column include the vertebrae interleaved with cartilagenous disc. The discs consist of a substantially hard outer ring of tissue called the annulus fibrosus, made of concentric layers of the crisscrossed fibrous cartilage which is predominantly collagen. Contained within the annulus fibrosus is a gelatinous highly elastic core, called the nucleus propulsus that has a high water content. The nucleus propulsus serves as the shock absorber of the spine to cushion compression loads on the vertebrae. The nucleus propulsus, together with the annulus fibrosus, also form a "ball bearing" within the spine to allow rotation and flexion. In the aging process, the water content in the nucleus propulsus typically decreases which makes the surrounding cartilage more deformable. Flexion of the spine causes uneven compression of the discs and can cause the nucleus propulsus to rupture or herniate the annulus propulsus. When a protrusion or herniation of the annulus propulsus impinges on a nerve root or the spinal cord, the patient may experience extreme pain in the back or legs. Such pain is often the first is sign of a disc disorder.
It is known in the art that mono-polar high-energy radiofrequency (Rf) energy probes can be used to shrink collagen in cartilagenous tissues (e.g., in joint capsule shrinkage procedures) where Rf is delivered to tissue between the mono-polar electrode at the end of a Rf probe and a groundplate. The prior art techniques involve "instantaneous" collagen shrinkage with a high energy Rf delivery (estimated 40 to 60 watts). It is unlikely that such a mono-polar Rf probe could be safe and effective for treating disc tissue, because thermal energy must be very precisely delivered. Such high energy mono-polar Rf probes suffer from several significant drawbacks.
A first disadvantage relates to the electrode dimension which is limited by the size (diameter) of the probe. When using a small diameter probe (e.g., 1.5 mm. to 3.0 mm. for percutaneous disc treatments) the electrode would have little surface area which means that Rf current intensity necessarily would be high (est. 40 to 60 watts) in order to emit Rf waves sufficient to perturb ions (to increase temperature) within a subsurface collagenous layer. Such high Rf energy levels would easily be capable of ablating or perforating surface of the tissue, which would be highly undesirable.
A second disadvantage of a high-energy mono-polar probe relates to the technique of its use which typically includes "painting" the hot probe tip across an overlying tissue layer or surface (e.g., a cartilage) in an effort to delivery thermal energy to a underlying collagenous layer. Again, typical Rf energy levels would necessarily be high (est. 40 to 60 watts) for the Rf to elevate the temperature of subsurface tissue layers since the probe is moving. It must be recalled that Rf energy causes thermal effects in biologic tissue by perturbation of ions as alternating Rf energy courses through the tissue in paths of least resistance between an active mono-polar Rf electrode and the groundplate. "Painting" the hot probe tip across a tissue surface causes the Rf paths through tissue to change constantly thus preventing the perturbation of ions in any particular path or location--thus preventing effective energy densities (temperature elevation) in any particular location to affect collagenous tissue. As can be seen in FIG. 1A, the Rf current paths are localized only momentarily and are not focused on the collagenous layer targeted for ionic perturbation. FIG. 1B shows the typical thermal gradient in tissue which is created by a high-energy mono-polar probe at a moment in time where higher temperatures are closer to the electrode. Thus, the prior art devices and methods must use a very high current intensity (to create high enough energy densities) to achieve "instantaneous" collagen shrinkage consistent with the painting technique.
What is needed is a technique and instrument for elevating the temperature of subsurface collagenous tissue (e.g., in a disc) that preferably (i) utilizes relatively low Rf power levels to prevent surface tissue ablation, (ii) utilizes bi-polar Rf flow (rather than mono-polar flow) to maintain the energy density in localized tissue (rather that between a mono-polar electrode and a remote groudplate); (iii) includes means to focus the paths of bi-polar Rf current flow within particular subsurface collagenous layers; (iv) includes means to create a more even thermal gradient between the tissue surface and the deeper collagenous tissue, and; (v) allows for observation of the shrinkage of collagenous tissue at a controlled rate that is slower than the prior art "instantaneous" rate of collagen shrinkage.